This invention relates to the separation of the luminance and chrominance components of a composite quadrature modulated color television signal, and more particularly to improved control of adaptive filtering to perform such a separation.
FIG. 1 is a graph of the frequency spectrum occupied by a typical quadrature modulated color television picture signal in accordance with the NTSC format. As is shown, luminance and chrominance information share the same spectrum. The luminance bandwidth is typically limited to 4.2 MHz, the I component bandwidth is typically limited to 1.3 MHz, and the Q component bandwidth is typically limited to 0.6 MHz. The I and Q components are present as carrier suppressed amplitude modulation components in phase quadrature upon a subcarrier at about 3.58 MHz. In the composite video signal this quadrature modulated subcarrier is added to the luminance carrier, and the resultant composite video signal is low pass filtered to 4.2 MHz.
The simplest means of separating the luminance and chrominance components of such a composite video signal is to apply the composite signal to a bandpass filter tuned to the subcarrier frequency, so that the chrominance signal is produced at the output of that filter. The chrominance signal can then be subtracted from the composite signal to produce the luminance signal. While this type of bandpass filter is inexpensive to build, especially in analog systems, it produces imperfect separation of the luminance and chrominance components. Luminance components that are in the chrominance band are interpreted as color, causing cross-color patterns that are typically perceived as a moving rainbow accompanying luminance transitions, or as other inappropriate color activity in the proximity of luminance details. Conversely, certain chrominance components are also interpreted as luminance, producing "dot-crawl" artifacts in the vicinity of the color transitions.
As will be further described in detail below, digital comb filters have been employed to produce more effective separation of the carrier and subcarrier contents. Filtering can be performed along the horizontal and vertical axes sequentially, to produce a chrominance signal that displays subcarrier activity along both of those axes. A more sophisticated filtering scheme for separating luminance and chrominance utilizes "adaptive" filtering to select either horizontal filtering only or filtering along both the horizontal and vertical axes.
FIG. 2 is a block diagram of a luma/chroma separation circuit that utilizes an adaptive filtering scheme. This circuit is typically used in a decoder, such as a decoder used in a studio setting. In this circuit the composite video signal input is applied to the input of a horizontal filter 12 and to delay element 8, the output of which is applied to the positive input of a summation circuit 10. The horizontal filter 12 is a bandpass filter centered around the subcarrier frequency.
Referring to FIG. 3, in its simplest form the horizontal filter 12 produces the weighted average of the composite video data associated with five consecutive pixels of the color video image. The clock signals associated with the television raster occur at four times the subcarrier frequency and the two orthogonal chrominance components are interleaved. Thus, in video according to the NTSC standard where the two video components are I and Q, a series of four pixels contains first a positive chrominance component of one type, e.g., +I, then a positive chrominance component of the other type, e.g., +Q, and then a pair of negative samples in the same sequence, i.e., -I followed by -Q. There also is a phase difference between successive lines of video, so that a two line sequence contains the following pattern of luminance and chrominance components: ##EQU1##
Thus, in the horizontal filter 12 shown in FIG. 3, only the pixel values corresponding to one chrominance component have non-zero coefficients; with the alternate pixels having zeros as their coefficients. In this example, the chrominance component that is currently being processed is the I component. One clock cycle later (with clocks occurring at four times the subcarrier frequency), the data will have advanced to the right and the filter will be processing the Q component instead. Because the smaller coefficients of the stages at the ends of the horizontal filter 12 have the opposite sign from the stage in the middle stage with the larger coefficient, the filter cancels out the luminance content (Y) of the composite signal, while it constructively adds the chrominance component that is currently being processed.
While this simple version of the horizontal filter has a less than ideal frequency response and produces some blurring of the chrominance detail, more complex versions of the filter can be constructed that have more stages and smaller coefficients, so that they operate according to the same general principles, but produce an output that has a more nearly ideal frequency response.
Referring again to FIG. 2, the output of the horizontal filter 12 is applied to a first line delay 14, to a first input of an adaptive control circuit 20, and to a first input of a vertical filter 18. The vertical filter 18 is also effectively a bandpass filter at the subcarrier frequency. The output of the first line delay 14 is applied to a second input of the vertical filter 18, to a second input of the adaptive control circuit 20, to the input of a second line delay 16, and to another delay 22 representing the delay associated with the vertical filter 18. The output of the second line delay 16 is applied to a third input of the vertical filter 18 and to a third input of the adaptive control circuit 20. The output of delay 22, which is the horizontally (only) filtered chrominance signal C.sub.H, is applied to a first input of a mixer 24, and the output of the vertical filter 18, which is a chrominance signal C.sub.VH that has been filtered both vertically and horizontally, is applied to a second input of the mixer 24. The mixer 24 is controlled by an output of the adaptive control circuitry 20 and produces a chrominance signal that is subtracted from the composite video input by summation circuit 10 to produce a luminance output.
Referring now to FIG. 4, the vertical filter 18 receives the same pixel of three consecutive lines, a last line L.sub.L, a present line L.sub.P, and a next line L.sub.N, and produces their weighted average according to the coefficients of the three stages. As with the horizontal filter 12, the opposite signs of the coefficients associated with the first and last stages of the vertical filter 18 cause any residual luminance Y.sub.R to be canceled out, while the chrominance components add constructively. And, as with the horizontal filter 12, the output alternates between I and Q chrominance components. And again, as with the horizontal filter 12, a more complicated version of the vertical filter 18 with more stages and more complex coefficients would produce a more nearly ideal frequency response.
In the circuit shown in FIG. 2 the adaptive control circuitry 20 makes a determination of how to control the mixer 24 based on the relationship between the output of the horizontal filter 12 over the same three consecutive lines that are inputs to the vertical filter 18. According to this control signal, the adaptive control circuit 20 and the mixer 24 select either a signal that has only been horizontally filtered 12 or one that has been both horizontally 12 and vertically 18 filtered. If the adaptive control signal is a single bit, this selection is between all one input or all the other. If the adaptive control signal contains multiple bits or is a linear analog signal, the selection can be various combinations of partly one input and partly the other.